Pigmented microporous silica microspheres

ABSTRACT

Pigmented silica microsphere having an average diameter from 2 μm to 100 μm and consisting essentially of from 10% to 70% by weight of particulate pigment, dispersed throughout a microporous silica matrix and, optionally, containing a coating of dense amorphous silica. The microspheres are produced by acidifying a water-in-oil emulsion. The pigmented silica microspheres are particularly useful as opacifying agents for coating compositions and as fillers for paper.

This is a division, of application Ser. No. 635,060, filed Nov. 25,1975, now U.S. Pat. No. 4,090,887.

BACKGROUND OF THE INVENTION

This invention relates to pigmented silica microspheres which are usefulas opacifying agents for coating compositions such as latex paints andas opacifiers and fillers for fibrous substrates such as paper.

A wide variety of porous silica particles are known in the art. Theporous silica particles are commonly designed for use as catalysts orcatalyst supports and less commonly for use as molding powders andinsulating materials. Porous silica particles used as catalysts orcatalyst supports are generally relatively large spherical particles,referred to as "beads," containing typical geltype pores having amaximum diameter of less than about 0.01μ. Porous silica particles ofthis type have been prepared by acidifying an aqueous solution of sodiumsilicate to form a silica gel, then agitating the gel in an oil atelevated temperatures to form silica "beads" or spherical silicaparticles, as described, for example, in U.S. Pat. Nos. 2,384,946 and2,528,767. Molding powders composed of porous silica microspheres havinga diameter in the range of 0.1μ to 50μ have been made by spray dryingsols containing colloidal silica, as described in U.S. Pat. No.3,301,635. The microspheres produced by this process are nonuniformaggregates of colloidal silica particles which, when compacted, havepores in the size ranges of 0.02μ to 5μ. Although the porosity of thecompacted silica particles provides some opacity, the opacity isinsufficient for pigmentary applications.

Hollow silica spheres having an average diameter from 100μ to 2500μ,useful as insulated materials, have been made by suspending particles ofalkali metal silicate and a metal oxide with a compound which liberatesa gas at elevated temperatures and heating the particles to 1000°-2500°F. (540°-1371° C.). The hollow silica spheres produced by this processare generally clear and transparent, rather than opaque, because thelarge air cell within the hollow sphere is too large to scatter lighteffectively.

A method for producing reticulated inorganic microparticles useful aspigment is described in U.S. Pat. No. 3,848,059. This process comprisesmixing two separate water-in-oil emulsions, each containing a separateinorganic salt to form spherical shaped microparticles of an insolublesalt having an average particle size which falls within the range of0.1μ to 5.0μ. The degree of porosity for these spherical microparticlesis insufficient for use as highly effective opacifying agents.

Highly porous inorganic microparticles having an average pore diameterof 0.03μ are described in U.S. Pat. No. 3,855,172. The inorganicmicroparticles are prepared by forming a sol of inorganic colloidalparticles in a polar liquid, forming a homogeneous solution of the solwith a polymerizable organic material, and polymerizing the organicmaterial. The organic material is burned off to form the porousmicroparticle. While the type of porosity which results renders theseparticles highly effective as catalyst supports, these particles areuseless as opacifying agents.

Generally, the porous silica microparticles formed by the process of theprior art are either large porous or hollow microspheres or very densemicroparticles which lack sufficient opacity for use in manyapplications.

This invention provides for a pigmented silica microsphere by a singleemulsion technique which exhibits high opacity and combines ease ofpreparation with economy.

SUMMARY OF THE INVENTION

According to the invention there is provided a pigmented silicamicrosphere having a diameter from 2 μm to 100μ and consistingessentially of from 10% to 70% by weight of particulate pigment, basedon the total weight of the microsphere, dispersed throughout amicroporous silica matrix, the silica matrix containing from 20% to 60%by volume of micropores, based on the total volume of the microsphere,the micropores having a median diameter of 0.004 μm to 0.05 μm. Ifdesired for added strength, the pigmented silica microspheres can have acoating of dense amorphous silica in an amount from 2% to 40% by weight,based on the total weight of the coated pigmented silica microsphere.The pigmented silica microsphere, coated or uncoated, is substantiallyspheroidal and, preferably, has a substantially smooth, continuoussurface. The average diameter of the microsphere is preferably from 2 μmto 50 μm when used as a paper filler and from 2 μm to 25 μm when used asan opacifying agent in paint.

The particulate pigment dispersed, preferably uniformly, throughout themicroporous silica matrix should preferably have a particle size from0.1 μm to 0.5 μm. A wide variety of organic and inorganic pigments maybe dispersed throughout the microporous silica matrix. Among the organicpigments which may be employed are metal phthalocyanines, such as copperphthalocyanine, substituted and unsubstituted quinacridones, benzidineyellows, toluidine yellows, and azo pigments. Inorganic colored pigmentssuch as lead chromate may also be useful. If desired, mixtures oforganic and inorganic pigment may also be employed in the microspheresof the invention. The maximum advantages of the invention are realizedif the pigment has a refractive index of at least 2. Since inorganicpigments commonly exhibit the desired high refractive index, they arepreferred. Although white lead, zinc oxide, zinc sulphide, siliconcarbide and various types of metal oxides and other inorganic materialscan be used, titanium dioxide is preferred. Such inorganic pigments canbe used alone or may be used in combination with organic pigments. Theinorganic pigments, such as TiO₂, can be uncoated or containconventional hydrous oxide coatings, such as silica, alumina, titaniaand the like.

The pigmented silica microspheres of the invention are prepared by thesteps of

(i) forming a water-in-oil emulsion by contacting a silaceous aqueousphase with an oil phase,

said silaceous aqueous phase consisting of water, particulate pigment,and at least one silicon compound selected from the group consisting ofcolloidal silica and alkali metal silicate,

said oil phase consisting of at least one water-immiscible hydrocarbonselected from the group consisting of liquid and low melting aliphatic,aromatic and chlorinated hydrocarbons,

(ii) adding acid to the water-in-oil emulsion formed in Step (i), theacid being added in an amount from 0.5 to 2.0 times the amount of acidneeded to react stoichiometrically with the total amount of base presentin the silaceous aqueous phase of Step (i) to gel the silaceous aqueousphase and form a slurry of pigmented silica microspheres, and

(iii) separating the pigmented silica microspheres from the slurryformed in Step (ii).

The pigmented silica microspheres can be separated from the slurry byconventional means such as centrifugation or filtration. To insure thatthe pigmented silica microspheres are freed from any residual oil whichmay remain on the surface of the microsphere, the separated microspheresof Step (iii) can be dispersed in an aqueous or alcoholic solution of ahydrophilic surfactant, separated from the solution of surfactant, andwashed with water or alcohol. By dispersing the microspheres in anaqueous or alcoholic solution iof hydrophilic surfactant any residualoil on the surface of the microspheres is dispersed in the surfactantsolution which is then separated from the microspheres by conventionaltechniques such as filtration. Washing the microspheres with water oralcohol insures the removal of any residual surfactant or salts formedduring processing which may remain on the surface of the microspheres.If aggregates of microspheres, which may form during the preparativeprocess, are observed during the washing process described above, it ispreferable at some stage in the washing process to pass the microsphereslurry through a screen, e.g., 325 mesh (Standard U.S. Sieve), to removethe aggregates.

The pigmented silica microspheres can be dried or left as a wet cakeprior to use without further processing. In applications requiringparticularly high strength and low friability, it is preferred that themicrospheres be coated with dense amorphous silica described, forexample, in U.S. Pat. No. 2,731,326. Pigmented silica microsphereshaving a coating of dense amorphous silica are prepared by theadditional steps of

(iv) forming an aqueous slurry of pigmented silica microspheres,

(v) adjusting the pH of the aqueous slurry of Step (iv) to from 8 to 11at a temperature from 60° C. to 125° C.,

(vi) contacting the aqueous slurry of Step (v) with an aqueous solutionof an alkali metal silicate in an amount sufficient to provide from 2%to 40% by weight of silica, calculated as SiO₂ and based on the totalsolids in the aqueous slurry of Step (v) and the aqueous solution,

(vii) adjusting the pH of the slurry of Step (vi) to less than 8,preferably 7.5, to deposit a coating of dense amorphous silica on thepigmented silica microspheres. The coated microspheres can be separatedfrom the aqueous slurry in the conventional manner without furthertreatment or can be subjected to curing by the additional step of

(viii) maintaining the slurry of step (vii) at a temperature of at least80° C. for at least 1 hour. The coating process is particularly usefulfor pigmented silica microspheres prepared using colloidal silica.

For applications requiring particularly high opacity, the opacifyingpower and porosity of the uncoated pigmented silica microspheres can beincreased by the additional steps of

(iva) forming an aqueous slurry of pigmented silica microspheres of Step(iii), and

(va) heating the aqueous slurry of Step (iva) at a temperature from 80°C. to 300° C., temperatures above the boiling point of the aqueousslurry requiring pressure to prevent boiling, and at a pH from 5 to10.7. This after-treatment is particularly useful for pigmented silicamicrospheres prepared using an alkali metal silicate.

DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic cross-sectional view of a typical singlepigmented silica microsphere of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a typical pigmented silica microsphere, 1, issubstantially spherical and has a diameter from 2 μm to 100 μm. Themicrosphere, 1, consists essentially of a silica gel or matrix, 2, whichcontains a plurality of micropores (not shown) having a median diameterfrom 0.004 μm to 0.05 μm. These micropores which are too small toeffectively scatter light, are not to be confused with vesicles or voidscommonly incorporated in materials to increase opacity. Unlike themicropores, vesicles or voids are at least 0.1 μm in diameter andtherefore sufficiently large to effectively scatter light. Dispersedthroughout the microporous silica matrix are a plurality of pigmentparticles, 3, having average particle sizes commonly from 0.1 μm to 0.5μm. The particulate pigment can have a wide variety of shapes, e.g.,from spherical anatase TiO₂ to acicular rutile TiO₂. Although themicropores are too small to effectively scatter light, the microporesenhance the light scattering power of high refractive index pigment,such as TiO₂, in the microsphere by lowering the average refractiveindex of the surrounding silica matrix to a value intermediate betweenthat of nonporous silica and air. In this way the particulate pigmentincorporated in the microporous silica matrix exhibits greater lightscattering efficiency than the same pigment embedded in a nonporouscomposition of the same refractive index as nonporous silica.

The pigmented silica microspheres can be prepared by forming awater-in-oil emulsion by mixing a silaceous aqueous phase and an oilphase, after which the silica is gelled by contact with acid.

The silaceous aqueous phase preferably contains from 5% to 40% by weightof silica, calculated as SiO₂ and based on the total weight of thesilaceous aqueous phase. The source of silica can be alkali metalsilicates such as sodium silicate or colloidal silica. Of the alkalimetal silicates, sodium silicates with a SiO₂ :Na₂ O weight ratio fromabout 2:1 to 4:1 are preferred. For example, when utilizing a 38% byweight solution of sodium silicate having a 3.25:1 SiO₂ :Na₂ O weightratio, it is preferred that one volume of the 38% solution be dilutedwith one volume of water prior to use. Colloidal silica is preferably inthe form of a colloidal silica sol containing from 10% to 40% by weightof silica having a particle diameter from 5 nm to 50 nm. In addition tothe source of silica and particulate pigment, the silaceous aqueousphase may also contain from 0.1% to 5% by weight of an anionic ornonionic dispersant, based on the weight of the particulate pigment, toprevent possible aggregation of the particulate pigment which may occurduring the acidification process which follows emulsification. Nonionicdispersants including water soluble polymeric dispersants, such aspolyvinylpyrrolidone, hydroxyethyl cellulose, polyvinyl alcohol, andamine type dispersants, may be used. Amine type dispersants, such as2-amino-2-methyl-1-propanol and 1,6-hexamethylenediamine, are preferred.

The oil phase should contain at least one water-immiscible hydrocarbonselected from liquid and low melting aliphatic, aromatic and chlorinatedhydrocarbons, e.g., cyclohexane, hexane, heptane, naptha, mineralspirits, xylene, chloroform and carbon tetrachloride. To insure theformation of a stable water-in-oil emulsion and uniformly sizedwater-droplets, the oil phase should preferably contain from 0.5% to 15%by weight of a nonionic water-in-oil emulsifier, based on the totalweight of the oil phase. Preferred water-in-oil emulsifiers includelyophilic fatty acid esters, ethoxylated alcohols and glycerides. In thesilaceous aqueous phase, it is preferable to include from 0.1% to 5% byweight of a soluble aluminum compound, based on the weight of the totalsolids in the silaceous aqueous phase and oil phase, in the oil phase.It is believed that the aluminum compound adsorbs on the surface of thesilicate microspheres and deactivates their surfaces, thus reducing anytendency to aggregate formation and adhesion to the blending equipment.The major criteria governing the selection of the aluminum compound areat least moderate solubility in organic media and inability to adverselyaffect the water-in-oil emulsion. Suitable soluble aluminum compoundsinclude aluminum acetate and aluminum isopropoxide.

In forming the water-in-oil emulsion, the ratio of the total volume ofthe silaceous aqueous phase to the total volume of the oil phase ispreferably from 1:6 to 1:1.

To gel the silica in the silaceous aqueous phase of the water-in-oilemulsion, acid is added to the emulsion in an amount from 0.5 to 2.0times the amount needed to react stoichiometrically with the totalamount of base present in the silaceous aqueous phase. A large excess ofacid is not recommended, since the presence of excess acid tends tobreak the water-in-oil emulsion. The acid is preferably oil- andwater-miscible, such as glacial acetic acid, trichloroacetic acid,chloroacetic acid, and formic acid. In practice, the acid can be addedalone or diluted with, for example, additional oil phase.

The pigmented silica microspheres can be separated from the emulsionslurry by conventional methods such as filtration, decantation, orcentrifugation. To insure that the microspheres are free of any residualoil phase which may adhere to their surface, the microspheres can beslurried in an aqueous or alcoholic solution of a surfactant, such as ahighly hydrophilic fatty acid ester or ethoxylated alcohol. Theresulting slurry can then be redispersed in water or alcohol and, ifdesired, filtered through a screen, e.g., 325 mesh (Standard U.S.Sieve), to remove any aggregates which may be present. The pH of thefiltered slurry is preferably adjusted to from 6 to 8.5 prior to removalof the microspheres therefrom by filtration and washing. The filteredmicrospheres can be dried or concentrated to a wetcake.

As described above, the resulting pigmented silica microspheres can beafter-treated to increase opacity by heating at elevated temperatures,i.e., 80° C. to 300° C., and preferably from 150° C. to 200° C., and ata pH from 5 to 10.7, preferably from 7 to 10.7. The period of time whichthe microspheres are subjected to these conditions can range from a fewminutes to several days, depending upon the temperature employed. Forexample, at 200° C. to 300° C. additional porosity is developed in a fewminutes, whereas at 80° C. to 100° C. several days are usually required.It is preferred that the microspheres by separated from the emulsionslurry in which they were prepared but not washed free of salts prior toforming the aqueous slurry which will be heated in accordance with theabove-described procedure, because the presence of soluble salts whichform upon acidification of the emulsion slurry tends to accelerate thedevelopment of additional porosity.

A coating of dense amorphous silica can be applied to the pigmentedsilica microspheres to increase strength and lower friability of themicrospheres. The dense amorphous form of silica is well-known in theart and differs from the microporous silica matrix in that the denseamorphous form exhibits substantially no porosity and no definitivestructure. The nature of dense amorphous silica and a variety of methodsfor its preparation are described, for example, in U.S. Pat. No.2,731,326.

The pigmented silica microspheres of the invention are highly useful asopacifying agents for a variety of coating compositions made with asolution vehicle. The microspheres perform most effectively when thenonvolatile components of the vehicle do not penetrate the microporousstructure, as in the case of latex paints the micropores are too smallto be penetrated by the latex particles. The microspheres may be addeddry, or preferably as an aqueous wetcake to a preformulated aqueouslatex emulsion. In a conventional latex paint the pigment volumeconcentration (PVC) is high enough to cause severe crowding of thepigment particles in the dry film, resulting in a significant loss inpigment scattering efficiency. Such crowding effects are reduced whenpigmented silica microspheres are used to opacify latex paints. Thepigment being uniformly distributed and fixed inside the silica matrixloses no scattering efficiency due to crowding at high microsphereloadings in the paint. For example, from 20% to 70% by volume ofmicrospheres, based on the total volume of solids in the paintcomposition, may be mixed with a typical starting paint or masterbatchemulsion formulated at about 34 PVC (pigment volume concentration) andcontaining about 3 lb./gal. (0.36 kilogram/liter) of TiO₂ with little orno extender. The microspheres are also useful as flatting agents forpaints, particularly in the size range above 5μ.

The microspheres of the invention are also useful as an opacifying agentand filler for fibrous substrates such as paper. In the paper industry,fillers such as TiO₂, clay or calcium carbonate are added to the paperfurnish prior to formation of the fibrous web on the paper-makingmachine. Since these fillers generally contain particles of less than 1μm in diameter, a substantial portion of the filler passes through thefibrous web and remains in the whitewater. The microspheres of theinvention should have higher retention than the common fillers becauseof their larger particle size and at the same time provide high opacity.

A further advantage of the microspheres is their light weight. Thedensity of the microspheres is substantially lower than conventionalfiller pigments, and consequently the microspheres are particularlyuseful for producing light weight paper with high opacity.

DESCRIPTION OF THE TESTS

In the experimental examples which follow paint compositions containingthe pigmented silica microspheres of the invention were tested foropacity (as measured by the scattering coefficient), scrub resistanceand stain resistance in accordance with the following procedures.

Scattering Coefficient

The scattering coefficient for a paint composition is a common measureof the opacity or hiding power of the paint per mil (0.0254 millimeter)of wet paint film thickness. To determine the scattering coefficient foreach paint tested, the paint is drawn on a Morest Chart Form 09, aglossy paper chart having black and white areas, using a draw-down bladewith a 0.002 inch (0.005 cm.) clearance, at 50% R. H. and 72° F. (22°C.). The film is allowed to dry overnight and reflectances over thewhite and black backgrounds are measured using a Gardner AutomaticMulti-purpose Reflectometer using a green filter. One reading is takenon each of two "white" and "black" sections of each of four charts. Thescattering coefficient is determined, using the above-measuredreflectances, from the appropriate Kubelka-Munk Chart (D. B. Judd,"Optical Specification of Light Scattering Materials," Journal ofResearch of the National Bureau of Standards, Vol. 19, 1937, p. 287).Scrub Resistance

The scrub resistance of each test paint is determined in accordance withthe procedure of ASTM method D 2486-69 T.

Test paint is applied to a black mylar panel with a 0.006 inch (0.015cm.) clearance blade. After aging one week, the coated panel is placedover a 1/2 inch (1.27 cm.) by 10 mil (0.025 cm.) shim and held in placeon a glass plate in a Gardner washability machine by means of a gasketedframe. It is then scrubbed with a hogshair bristle brush and an abrasivescrub medium until failure occurs over the shim. the number of scrubcycles to failure is noted.

Stain Resistance

To determine the stain resistance of the test paints, side-by-sidedraw-downs of a control paint and a test paint are made on white Morestcharts using a draw-down blade having a 0.006 inch (0.015 cm.)clearance. After allowing the draw-downs to dry four days, equal areasof the side-by-side films are covered with an excess of a heavy,oil-based ink. After 90 seconds, the ink is wiped off as completely aspossible with paper towels. The degree of staining (darkening of thefilm) is visually determined and the test paint is rated better or worsethan the control. A "plus" designation indicates better stain resistanceand a "minus" designation indicates worse stain resistance than thecontrol. The degree of contrast relative to the control is ranked as oneof the following: "equal, very slight, slight, considerable anddefinite."

The following examples illustrate the invention.

EXAMPLE 1

A silaceous aqueous phase is prepared by mixing 100 ml. of acommercially available colloidal silica sol containing 30% by weight ofcolloidal silica, sold by Du Pont under the name Ludox SM, 3.3 ml. of70% by weight 1,6-hexamethylenediamine, and 30 g. of a commerciallyavailable TiO₂ pigment, sold by Du Pont under the name Ti-Pure R-900, ina laboratory mixer for five minutes.

A water-in-oil emulsion is prepared by agitating the above-preparedaqueous phase in 400 ml. of an oil phase containing one volume ofcyclohexane, one volume of chloroform, and 0.2 volume of sorbitanmonooleate (a nonionic water-in-oil emulsifying agent sold by ICIAmerica under the name Span 80). Agitation is continued for threeminutes using a blender fitted with a one quart (0.946 liter) blenderjar and speed control to effect emulsification. While continuing theagitation, 200 ml. of the oil phase described above containing 1.5 ml.of glacial acetic acid is added and agitation is continued for threemore minutes.

The resulting pigmented silica microspheres are then centrifuged toseparate them from the oil phase. To insure the removal of any residualoil phase, the microspheres are dispersed in an excess of a 50% byvolume aqueous solution of a hydrophilic fatty acid ester (a nonionicoil-in-water emulsifying agent sold by ICI Inc. under the name Tween20), centrifuged and redispersed in water.

The above procedure is repeated several times to accumulate a sufficientquantity of microspheres for testing in paint compositions. Theaccumulated microsphere slurries are filtered through a 325 mesh(Standard U.S. Sieve) screen, then neutralized to pH 7 with glacialacetic acid. The resulting slurry is filtered and washed with water to aspecific resistance of 60,000 Ω-cm. The solids content of the wetfiltercake (wetcake) is 43.5%. A sample of the wetcake is dried at 110°C. and found to contain 94.25% by weight of ash of which 40.7 % byweight is TiO₂ and 44.9% by weight is SiO₂. The dried microspheresexhibit a surface area of 116 m.² /g. as measured by nitrogen adsorpsionand a solids density of 2.5 g./cc. as measured by liquid pycnometry.Examination of the dried microspheres under a light microscope revealsopaque spheres having diameters from 2 μm to 20 μm. From nitrogenadsorption analysis, the porosity of the microspheres is found to beapproximately 45% by volume.

The pigmentary properties of the microspheres are evaluated by addingthe wetcake to a preformulated latex paint masterbatch 100 gallons(45.35 l.) of which contains 310.6 lb. (139.8 kg.) of a commerciallyavailable TiO₂ pigment, sold by Du Pont under the name Ti-Pure R-931,and 334.1 lb. (150.3 kg.) of a conventional emulsion paint vehicleconsisting of a 55% by weight solids polyvinyl acetate copolymeremulsion. Therefore, the masterbatch contains 3.1 lb. of TiO₂ /gal.(0.27 kg./liter) and has a pigment volume concentration (PVC) of 34%,which is below the critical PVC.

The test paints are prepared by adjusting the pH of the masterbatch to8.5 with ammonia then adding an amount of the microsphere wetcake togive the microsphere volume concentrations or "bead" volumeconcentrations (BVC) shown in Table I, paints T1-1 through T1-7. The BVCis determined from the following relationship: ##EQU1##

As a control, a conventional emulsion paint is prepared using the sameingredients as in the masterbatch but with extenders to raise the PVCabove critical. One hundred gallons (45.35 liters) of the control paintcontains the same amount of TiO₂ as the masterbatch, 57.4 lb. (25.8 kg.)of silica extenders, 62.1 lb. (27.9 kg.) of calcium carbonate extender,and 281.6 lb. (127.7 kg.) of the masterbatch emulsion vehicle.Therefore, the control paint contains 3.1 lb. of TiO₂ /gal. (0.27kg./liter) and has a PVC of 50%. The properties of the control, referredto as C-1 are shown in Table I.

In addition to showing the high opacity of the paint compositionscontaining the pigmented silica microspheres of the invention, the datain Table I shows an additional feature of the invention, that is, thehigh bulking of the microspheres. In other words, the addition of themicrospheres to the masterbatch substantially increases the final volumeof the paint. Conventional opacifying pigments, such as TiO₂, arerelatively dense (low bulking) and contribute little to the paintvolume. Table I shows that the hiding power and the film properties ofthe test paints, T1-1 through T1-7, match the Control, C-1, at about aBVC of 40%. The stain resistance in all cases is at least as good as theControl, C-1.

                                      TABLE I                                     __________________________________________________________________________    Example 1   Scattering                                                                          Scrub  Stain  Relative Volume                               Test Paint                                                                           BVC, %                                                                             Coefficient                                                                         Resistance                                                                          Resistance                                                                            of Paint                                      __________________________________________________________________________    Masterbatch                                                                           0   2.13  620   Definite +                                                                            100                                           T1-1   20   3.09  440   Considerable +                                                                        117                                           T1-2   30   3.60  320   Slight +                                                                              129                                           T1-3   40   3.88  240   Very Slight +                                                                         145                                           T1-4   50   4.29  160   Equal   167                                           T1-5   60   4.96  140   Equal   200                                           T1-6   65   4.62  120   Equal   224                                           T1-7   70   4.52  100   Equal   256                                           C-1     0   3.87  260   Control 100                                           __________________________________________________________________________

EXAMPLE 2

The procedure of Example 1 is followed except that the oil phasecontains two volumes of carbon tetrachloride, three volumes ofcyclohexane, and 0.5 volume of the sorbitan monooleate described inExample 1.

The resulting microspheres exhibit substantially the same appearance asthe microspheres of Example 1 when viewed under a light microscope.

EXAMPLE 3

Pigmented silica microspheres are prepared and isolated following theprocedure of Example 1. A filtered and washed wetcake containing about400 g. of the microspheres is slurried in two liters of water. Theslurry is heated to 90° C. and held at that temperature while stirringslowly with a paddle blade mixer. The pH of the slurry is adjusted to9.5 with 50% by weight aqueous solution of sodium hydroxide. Then 300ml. of an aqueous solution containing one volume of 29% by weightsolution of sodium silicate (SiO₂ /Na₂ O = 3.25, Du Pont Technical GradeNo. 9) and one volume of water is rapidly added to the slurry from a 500ml. dropping funnel. The pH of the slurry after the sodium silicateaddition is 10.1. To deposit a dense amorphous silica coating on themicrospheres, about 310 ml. of a one molar aqueous solution of sulfuricacid is added to the slurry at a rate of about 9 ml./min. over a periodof 34 minutes. The pH of the resulting slurry is 7.5. The slurry is thenheated to 90° C. and held at that temperature for one hour. The denseamorphous silica coated microspheres are recovered by filtration andwashed to a specific resistance of 22,000 Ω-cm. The wetcake solidscontent is 49.5%. Analysis of the dried sample, as described in Example1, shows 94.6% of ash, of which 62.3% by weight is SiO₂ and 34.9% byweight is TiO₂. The dried product has a surface area of 106 m.² /g. andwhen viewed under a light microscope shows a substantially similarappearance to the microspheres of Example 1.

To test the pigmentary properties of the dense amorphous silica coatedmicrospheres, masterbatch, control, and test paint compositions areprepared in accordance with the procedure described in Example 1. Theproperties of the test paints are shown in Table II, wherein C-2designates the Control and T2-1 through T2-7 designate the test paints.The data in Table II shows that the dense amorphous silica coatingimproves scrub resistance, i.e., lowers friability, with substantiallyno sacrifice in opacity. Thus a microsphere paint formulated at a BVC of40% (T2-3) matched the Control C-2 in opacity but exceeded it in filmproperties.

                                      TABLE II                                    __________________________________________________________________________    Example 2   Scattering                                                                          Scrub Stain   Relative Volume                               Test Paint                                                                           BVC, %                                                                             Coefficient                                                                         Resistance                                                                          Resistance                                                                            of Paint                                      __________________________________________________________________________    Masterbatch                                                                           0   2.31  630   Definite +                                                                            100                                           T2-1   20   3.19  520   Considerable +                                                                        114                                           T2-2   30   3.79  430   Slight +                                                                              123                                           T2-3   40   4.03  360   Very Slight +                                                                         136                                           T2-4   50   5.03  260   Equal   154                                           T2-5   60   5.89  260   Equal   181                                           T2-6   65   5.77  210   Equal   201                                           T2-7   70   5.77  200   Equal   227                                           C-2     0   3.98  340   Control 100                                           __________________________________________________________________________

EXAMPLE 4

To a one quart (0.946 liter) blender jar are added 500 ml. of an aqueoussolution containing one volume of water and one volume of a 29% byweight solution of sodium silicate (SiO₂ /Na₂ O = 3.25, Du PontTechnical Grade No. 9) and 3.0 g. of 2-amino-2 -methyl-1-propanol. Whilemixing the solution, 100 g. of a commercially available TiO₂ pigmentsold by Du Pont under the name Ti-Pure R-100 is added to the solution.Mixing is continued for five minutes. Then 250 ml. of the resultingslurry is transferred to a 2000 ml. plastic beaker.

To form a water-in-oil emulsion 1000 ml. of an oil phase consisting ofone volume of chloroform, one volume of cyclohexane, 0.2 volume of thesorbitan monooleate described in Example 1, and 2.5 g. of aluminumacetate (basic) is added to the beaker containing the silaceous aqueousphase prepared above. The emulsification is effected by mixing for 3minutes with a Gifford-Wood homogenizer-mixer. While continuing themixing 200 ml. of the oil phase (with the aluminum acetate) containing27.0 ml. of glacial acetic acid is poured into the emulsion and mixingis continued for three more minutes.

The resulting microspheres are isolated as described in Example 1 andwashed to a specific resistance of 20,000 Ω-cm. A wetcake of 56.0% byweight of solids is obtained. Analysis of a dried sample shows 75.5% ofash of which 48.6% is TiO₂ and 50.0% is SiO₂. The surface area of thedried microspheres is 83 m.² /g. and the solids density is 2.05 g./cc.From nitrogen adsorption analysis the porosity is found to be about 35%by volume. Upon examination under a light microscope, the microspheresexhibit diameters from 5 μm to 10 μm.

To determine the pigmentary properties of the pigmented silicamicrospheres, masterbatch, test samples, and a control are prepared asdescribed in Example 1. The data, shown in Table III, illustrates thatthe opacifying power of these microspheres is not as great as thoseprepared using colloidal silica as described in Example 1, due to lowerporosity. However, the film properties are good, matching the controlC-4 at a BVC of 50% and acceptable as high as a BVC of 65%.

                                      TABLE III                                   __________________________________________________________________________    Example 4   Scattering                                                                          Scrub Stain   Relative Volume                               Test Paint                                                                           BVC, %                                                                             Coefficient                                                                         Resistance                                                                          Resistance                                                                            of Paint                                      __________________________________________________________________________    Masterbatch                                                                           0   2.16  610   Definite +                                                                            100                                           T4-1   20   2.60  590   Considerable +                                                                        112                                           T4-2   30   2.90  530   Considerable +                                                                        121                                           T4-3   40   3.12  450   Slight +                                                                              132                                           T4-4   50   3.30  340   Very slight +                                                                         148                                           T4-5   60   3.48  250   Very Slight +                                                                         172                                           T4-6   65   3.42  200   Equal   189                                           C-4     0   4.21  340   Control 100                                           __________________________________________________________________________

EXAMPLE 5

Four hundred grams of pigmented silica microspheres prepared asdescribed in Example 4 are washed once (specific resistance 130 Ω-cm.)and filtered through a screen as described in Example 1. The resultingmicrosphere slurry is diluted to a volume of three liters with water andadded to a five-liter, four-neck, round-bottom flask fitted with astirrer, thermometer, reflux condenser, and heating mantle. The slurryis heated to 100° C. while slowly stirring and allowed to reflux for oneday at pH 7.2. The pH is then raised to 9 with an aqueous solution of50% by weight of sodium hydroxide. After refluxing for two more days,the slurry is cooled and the pH adjusted to 7 with glacial acetic acid.The slurry is filtered and washed to a specific resistance of 30,000Ω-cm. The wetcake solids content is 53.6% by weight. Analysis of a driedsample shows 95.8% by weight of ash, of which 48.0% by weight is TiO₂and 51.1% by weight is SiO₂. The surface area of the dried microspheresis 62 m.² /g. and solids density is 2.51 g./cc. The porosity vianitrogen adsorption is 48% by volume.

To determine the pigmentary properties of the pigmented silicamicrospheres, masterbatch, test samples and a control are prepared asdescribed in Example 1. The data, shown in Table IV, illustrates thatthe higher internal porosity of these after-treated microspheres resultsin greater hiding power than the microspheres of Example 4.

                                      TABLE IV                                    __________________________________________________________________________    Example 5   Scattering                                                                          Scrub Stain   Relative Volume                               Test Paint                                                                           BVC, %                                                                             Coeffient                                                                           Resistance                                                                          Resistance                                                                            of Paint                                      __________________________________________________________________________    Masterbatch                                                                           0   2.16  600    --     100                                           T5-1   24   2.94  580   Slight +                                                                              115                                           T5-2   35   3.35  450   Very Slight +                                                                         125                                           T5-3   45   3.70  320   Equal   139                                           T5-4   56   4.06  250   Very Slight -                                                                         159                                           T5-5   65   4.11  190   Slight -                                                                              188                                           T5-6   70   4.37  170   Slight -                                                                              209                                           T5-7    0   4.21  340   Control 100                                           __________________________________________________________________________

EXAMPLE 6

The procedure of Example 5 is followed except that after the first washand filtration the microsphere slurry is diluted to two liters withwater, the pH is adjusted to 9, and the slurry is heated to 150° C. forthree hours in an autoclave. The slurry is then cooled, the pH adjustedto 7, and washed to a specific resistance of 20,000 Ω-cm. The wetcakesolids content is 43.0% by weight and the dried microspheres contain94.3% by weight of ash, of which 50.1% by weight is TiO₂, and 50.6% byweight is SiO₂. The surface area is 46 m.² /g.; solids density, 256g./cc.; and porosity, 41% by volume.

The pigmentary properties for the microspheres in test paints versus themasterbatch and control, prepared as described in Example 1, are shownin Table V.

                                      TABLE V                                     __________________________________________________________________________    Example 6   Scattering                                                                          Scrub Stain   Relative Volume                               Test Paint                                                                           BVC, %                                                                             Coefficient                                                                         Resistance                                                                          Resistance                                                                            of Paint                                      __________________________________________________________________________    Masterbatch                                                                           0   2.41  990   Definite +                                                                            100                                           T6-1   21   2.80  530   Slight +                                                                              120                                           T6-2   32   3.36  510   Very Slight +                                                                         134                                           T6-3   42   3.74  360   Very Slight -                                                                         153                                           T6-4   52   4.28  260   Slight -                                                                              179                                           T6-5   62   4.17  140   Considerable -                                                                        219                                           C-6     0   3.68  500   Control 100                                           __________________________________________________________________________

EXAMPLE 7

The procedure of Example 6 is followed except that the microsphereslurry is at pH 7 and is heated to 150° C. for 15 minutes. The resultingwetcake contains 40.5% by weight of solids. The dried microspherescontain 88.3% by weight of ash, of which 49.6% by weight is TiO₂, and51.3% by weight is SiO₂. The surface area of the microspheres is 79 m.²/g.; solids density, 2.45 g./cc.; and porosity, 48% by volume.

The pigmentary properties of the microspheres, shown in Table VI, aredetermined from masterbatch, test paints, and a control prepared asdescribed in Example 1.

                  TABLE VI                                                        ______________________________________                                        Example 7           Scattering                                                                              Relative Volume                                 Test Paint                                                                             BVC, %     Coefficient                                                                             of Paint                                        ______________________________________                                        Masterbatch                                                                             0         2.29      100                                             T7-1     23         2.48      120                                             T7-2     34         2.90      134                                             T7-3     44         3.48      153                                             T7-4     54         3.83      180                                             T7-5     64         4.05      221                                             T7-6     69         3.89      249                                             C-7       0         3.55      100                                             ______________________________________                                    

What is claimed is:
 1. Pigmented silica microsphere having a diameterfrom 2μ to 100μ and consisting essentially of from 10% to 70% by weightof particulate pigment, based on the total weight of the microsphere,dispersed throughout a microporous silica matrix, the silica matrixcontaining from 20% to 60% by volume of micropores, based on the totalvolume of the microsphere, the micropores having a median diameter from0.004 μm to 0.05 μm.
 2. Pigmented silica microsphere according to claim1 having a coating of dense amorphous silica in an amount from 2% to 40%by weight, based on the total weight of the coated microsphere. 3.Pigmented silica microsphere according to claim 1 having an averagediameter from 2 μm to 50 μm.
 4. Pigmented silica microsphere accordingto claim 1 having an average diameter from 2 μm to 25 μm.
 5. Pigmentedsilica microsphere according to claim 1 wherein the particulate pigmentis an inorganic pigment having a refractive index of at least
 2. 6.Pigmented silica microsphere according to claim 5 wherein the inorganicpigment has a particle size from 0.1 μm to 0.5 μm.